The present invention relates to a method for detecting the failure of nuclear reactor fuel. It is generally available for the detection of fuel failure, providing confirmation as to whether a fuel assembly used in a nuclear fission reactor ("nuclear reactor") is failed or not and, if failed, capable of identifying failed fuel element and the scale of the failure. The invention relates in particular to a method for detecting the failure of a nuclear reactor fuel by means of emission computer tomography (ECT) utilising the radioactivity of the fuel itself. The method is available not only for fuel assemblies in fast reactors, light water reactors, heavy water reactors and other nuclear reactors but can also be extensively used for inspection to identify sources of failure.
A method for identifying the failure of a nuclear reactor fuel assembly comprises the following steps: (1) to collect information on the occurrence of failure in a nuclear reactor; (2) to identify the approximate location of the failure in the reactor core; and (3) to identify failed fuel assemblies.
(1) Collection of information on the occurrence of failure in a nuclear reactor
(a) CG method (cover gas method)
In a fast reactor, liquid sodium is used as a coolant and argon gas fills the space above the liquid sodium as a cover gas. When a fuel assembly fails, fission product (F.P) gas is released, and this will be contained in the cover gas. Therefore, it is possible to detect failure of the fuel assembly by sampling the cover gas and finding out whether the F.P gas is present in the cover gas or not.
(b) DN method (delayed neutron method)
When a fuel assembly fails, nuclides such as iodine, bromine, etc., which emit delayed neutrons, migrate into the liquid sodium. Thus, it is possible to detect failure of the fuel assembly by sampling the sodium with nuclides which delayed neutrons emit.
(2) Identification of the approximate position of a failed assembly in a reactor core
(a) FLT method (flux tilting method)
A fast reactor is operated by passing a primary system coolant in a plurality of loops. When a fuel assembly fails somewhere in the reactor core, nuclides which emit neutrons reach the primary system loop. In this case, there is a difference in the duration of time taken to reach the loop which depends upon the location of the failed assembly in the core. Therefore, it is possible to identify approximately in which zone the failure has occurred by sampling the liquid sodium in each loop and finding out the time difference for the neutrons to reach the loop.
(b) S/V method (selector valve method)
A plurality of ducts for sampling are provided in a nuclear reactor, and by sampling liquid sodium from each of the ducts by means of a pump, it is possible to identify approximately where the fuel failure has occurred.
(3) Method to identify a failed fuel assembly
(a) TG method (tag gas method)
Different types of special gas are placed in each of the fuel assemblies, and so, by detecting the gas coming out of a failed fuel assembly, it is possible to detect in which fuel assembly the failure has occurred.
(b) Sipping method
A cap is placed on the head of each fuel assembly through which liquid sodium is sipped by means of a pump, and it is possible to identify a failed fuel assembly by finding out whether the F.P gas is contained or not.
By the methods (1)-(3) described above, the failed fuel assembly can be ultimately identified. To identify further in which fuel element in the fuel assembly the failure has occurred, it has been the practice in the past to disassemble each fuel assembly and to perform post-irradiation tests on each of the fuel elements.
Since this must be carried out for each of the fuel elements, it is a very inefficient method and much time and labor are required before the failure is finally confirmed.